When connecting a cable etc. for supplying electric power to an electric power device etc., a plug-in connector is often used. A plug-in connector is one which a plug, such as a pin plug to which a cable is connected, for example, is inserted into a socket, such as a receptacle or a socket plug, and thereby they are connected.
In patent document 1 and 2, examples wherein, on a plug or a socket, one set of keys arranged separately along circumferential direction is constructed, and on the socket or the plug, one set of key grooves is arranged along circumferential direction so as to correspond to the one set of keys, in order to prevent wrong insertion of the plug to the socket, are disclosed.
When inserting the plug into the socket, the position of the one set of keys is aligned with the position of one set of key grooves of the socket. Thus, only when the patterns of keys and key grooves match, the plug can be inserted into the socket.